Avalonia was a microcontinent in the Paleozoic era.
Crustal fragments of this former microcontinent underlie south-west Great Britain, and the
eastern coast of North America.
It is the source of many of the older rocks of Western Europe, Atlantic Canada, and parts
of the coastal United States.
Avalonia is named for the Avalon Peninsula in Newfoundland.
Avalonia developed as a volcanic arc on the northern margin of Gondwana.
It eventually rifted off, becoming a drifting microcontinent.
The Rheic Ocean formed behind it, and the Iapetus Ocean shrank in front.
It collided with the continents Baltica, then Laurentia, and finally with Gondwana, ending
up in the interior of Pangea.
When Pangea broke up, Avalonia's remains were divided by the rift which became the Atlantic
Ocean.
Development
The early development of Avalonia is believed to have been in volcanic arcs near a subduction
zone on the margin of Gondwana.
Some material may have accreted from volcanic island arcs which formed further out in the
ocean and later collided with Gondwana as a result of plate tectonic movements.
The igneous activity had started by 730 million years ago and continued until around 570 million
years ago, in the late Neoproterozoic.
In the early Cambrian, the supercontinent Pannotia broke up and Avalonia drifted off
northwards from Gondwana.
This independent movement of Avalonia started from a latitude of about 60° South.
The eastern end of Avalonia collided with Baltica, a continental plate occupying the
latitudes from about 30°S to 55°S, as Baltica slowly rotated anticlockwise towards it.
This happened at the end of the Ordovician and during the early Silurian.
In the late Silurian and lower Devonian, the combined Baltica and Avalonia collided progressively,
with Laurentia, beginning with the long extremity of Avalonia which is now attached to the USA
and Canada.
The result of this was the formation of Euramerica.
At the completion of this stage, the site of Britain was at 30°S and Nova Scotia at
about 45°S.
This collision is represented by the Caledonian folding or in North America as an early phase
in the Acadian orogeny.
In the Permian, the new continent and another terrane, Armorica which included Iberia, drifted
in from Gondwana, trapping Avalonia between it and the continent so adding Iberia/Armorica
to Euramerica.
This was followed up by the arrival of Gondwana.
The effects of these collisions are seen in Europe as the Variscan folding.
In North America it shows as later phases of the Acadian orogeny.
This was happening at around the Equator during the later Carboniferous, forming Pangaea with
Avalonia near its centre but partially flooded by shallow sea.
In the Jurassic, Pangaea split into Laurasia and Gondwana, with Avalonia as part of Laurasia.
In the Cretaceous, Laurasia broke up into North America and Eurasia with Avalonia split
between them.
Iberia was later rotated away again as the African part of Gondwana brushed past.
This last movement caused the Alpine orogeny including the raising of the Pyrenees during
the Miocene and Pliocene.
As a result of this, part of Avalonia is now to be found on each side of the Straits of
Gibraltar.
Consequences
The Avalonian part of Great Britain almost exactly coincides with England and Wales.
Elsewhere in Europe, parts of Avalonia are found in the Ardennes of Belgium and north-eastern
France, north Germany, north-western Poland, south-eastern Ireland, the south-western edge
of the Iberian Peninsula.
Part of the British-Belgian section formed an island in the Carboniferous, affecting
the disposition of coalfields; this is known by names such as the 'London-Brabant Island'.
Its bulk had an effect on the geological structure between the Ardennes and the English Midlands
by influencing the subsequent crustal folding resulting from the Variscan collision.
In Canada, Avalonia comprises the Avalon Peninsula of southeast Newfoundland, southern New Brunswick,
part of Nova Scotia, Prince Edward Island.
In the United States, Avalonia comprises northern coastal Maine, all of Rhode Island, and other
sections of coastal New England.
See also Mistaken Point
References
External links Map showing Avalonia in the Ordovician Period
Relative positions of Avalonia and Gondwana